What role do organizational preconditions play in the constitution of expertise? This is the guiding question for this paper, which studies how expertise is shaped in the Intergovernmental Science–Policy Platform on Biodiversity and Ecosystem Services (IPBES). By organizing the world’s experts on biodiversity, IPBES sets out to produce policy-relevant knowledge. However, in contrast to many other international expert bodies such as the IPCC, IPBES assesses not only scientific knowledge, but also other forms of knowledge, including indigenous and local knowledge. In light of IPBES’s ambition to become an epistemic authority by synthesizing heterogeneous knowledge forms, it is of great interest to investigate how this expertise is constructed. What does ‘expertise’ mean for IPBES, and how are experts selected? Based on documents studies, this study explores the organizational structure through which IPBES assesses and selects experts. The analysis finds that the construction of expertise involves scientific as well as political dimensions. In the conclusions, problems are raised that are related to the outcome of this process and may threaten the epistemic authority of IPBES.